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<title>THE BOOST MPL LIBRARY: apply/lambda Semantics</title>
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<div class="section" id="apply-lambda-semantics">
<h1><a class="toc-backref" href="./changes-in-boost-1-32-0.html#id64" name="apply-lambda-semantics"><tt class="literal"><span class="pre">apply</span></tt>/<tt class="literal"><span class="pre">lambda</span></tt> Semantics</a></h1>
<ol class="arabic">
<li><p class="first">Nullary metafunction classes are now consistent with all other
metafunction classes, rather than being identical to nullary
metafunctions:</p>
<table border="1" class="table">
<colgroup>
<col width="37%" />
<col width="22%" />
<col width="40%" />
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<tr><th><p class="first last">Nullary...</p>
</th>
<th><p class="first last">Before</p>
</th>
<th><p class="first last">Now</p>
</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><p class="first last">Metafunction</p>
</td>
<td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
</td>
<td><p class="first last"><tt class="literal"><span class="pre">f::type</span></tt></p>
</td>
</tr>
<tr><td><p class="first last">Metafunction class</p>
</td>
<td><p class="first last"><tt class="literal"><span class="pre">c::type</span></tt></p>
</td>
<td><p class="first last"><tt class="literal"><span class="pre">c::apply&lt;&gt;::type</span></tt> or
<tt class="literal"><span class="pre">c::apply::type</span></tt> <a class="footnote-reference" href="#id21" id="id20" name="id20">[7]</a></p>
</td>
</tr>
</tbody>
</table>
<p>This fixes a flaw in the metafunction class/metafunction algebra 
that long has been a major source of other inconsistencies throughout 
the library.</p>
</li>
<li><p class="first">The <tt class="literal"><span class="pre">apply</span></tt> metafunction now automatically invokes <tt class="literal"><span class="pre">lambda</span></tt>
on its first argument.  For example, the following is well-formed now:</p>
<pre class="literal-block">
typedef apply&lt; plus&lt;_1,_1&gt;, int_&lt;2&gt; &gt;::type four;
</pre>
<p><tt class="literal"><span class="pre">apply_wrap</span></tt><em>n</em> auxiliary metafunctions are available for rare cases 
when it's crucial to eliminate the small overhead associated with lambda 
expressions while still avoiding various compiler bugs known to
cause problems with
the obvious <tt class="literal"><span class="pre">f::template</span> <span class="pre">apply&lt;...&gt;::type</span></tt> syntax.</p>
</li>
<li><p class="first">The <tt class="literal"><span class="pre">lambda</span></tt> implementation for broken compilers has been
reworked to emulate the semantics of the full-fledged version
as closely as possible; consequently, any code out there that
was taking advantage of the semantic quirks of less complete
emulation in the previous release may stop working with the new
one.</p>
</li>
</ol>
<p>To get a detailed picture of the new apply/lambda semantics, please
see the test case at
<a class="reference" href="../../test/apply.cpp" target="_top">libs/mpl/test/apply.cpp</a>
and the corresponding sections of the MPL <a class="reference" href="./reference-manual.html">reference manual</a>.</p>
<table class="footnote" frame="void" id="id21" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id20" name="id21">[7]</a></td><td>The second form is available only on fully conforming compilers.</td></tr>
</tbody>
</table>
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